In many situations it is either desirable or necessary to separate solids or other materials from liquids. For example, in aquaculture ponds, there tends to be, over a period of time, an increase in solid matter in the pond water which may be undigested food, excrement from fish or crustaceans being grown in the ponds or any other form of organic and inorganic solid material. As a result of the build up of this solid matter, it is necessary to empty the pond at regular intervals usually at harvest time which creates an environmental problem in liquid disposal. Further heavy machinery is usually required to be used at considerable cost to remove and spread the solid matter which has settled as sludge on the base of the pond. The pond is then required to be refilled necessitating use of a large quantity of water. Where the aquaculture system uses fresh water, refilling of the pond provides a drain on natural resources and therefore is becoming environmentally unacceptable. It would be advantageous therefore to reduce the frequency at which changing the pond water in aquaculture systems is undertaken. It would also be advantageous to remove or contain the sludge in the effluent water from agricultural ponds to reduce the effects of solids discharge into the natural waterways and contain and/or remove the sludge or solid wastes from with the pond prior to discharge.